In the last years laser metal forming (LMF) has been introduced in industrial manufacturing. Laser metal forming is a process where a high power laser locally melts a focussed stream of metal powder or a metal wire onto a substrate. In this way material can be added to the underlying part. The method is suitable for controlled material build-up and the laser generated parts are characterised by a dense microstructure which is usually free of pores.
Laser metal forming has been recently used for commercial manufacturing of superalloy components due to its attractive potential for repair of locally damaged or worn parts. Indeed, it is possible to selectively add material at desired locations and to re-establish the full functionality of a component. It is clear that laser repair technology is particularly attractive for the refurbishment of expensive parts that are affected by local damage or local mechanical wear. Turbine or compressor components such as blades and vanes are typical examples.
During laser metal forming substrate material is locally molten and powder (or wire) is injected into the melt pool with a suitable powder (or wire) feeder mechanism. After a certain interaction time (which is determined by the laser spot size and the relative movement between laser and substrate) the molten material resolidifies leading to material build-up on the substrate.
So far, several patents have been issued for the laser metal forming process. The basic principle is described in EP-A1-0 558 870, DE-C1-199 49 972, DE-C1-198 53 733, U.S. Pat. No. 5,873,960, U.S. Pat. No. 5,622,638 or U.S. Pat. No. 4,323,756.
The application of epitaxial material build-up for protective coatings is covered by U.S. Pat. No. 6,277,500, applications for generation or refurbishment of single crystal components are described in U.S. Pat. No. 6,024,792, EP-A1-0 740 977, WO95/35396 or U.S. Pat. No. 5,914,059. Except U.S. Pat. No. 6,024,792 none of these patents mentions the significance of temperature information from the melt pool in order to obtain a desired microstructure. U.S. Pat. No. 6,024,792 states that the laser power has to be set in a way to obtain adequate values for temperature gradient G and solidification velocity Vs, in order to obtain epitaxial growth on the substrate, however does not suggest a method for automatic laser power control in order to obtain a specific microstructure which results in optimum hardness or wear properties or for avoiding melt pool convections.
Another patent application, WO95/06540 suggests the use of a pyrometer for interactive laser welding of super alloy articles measuring the substrate pre-heating temperature.
The collection of optical signals from the melt pool is also depicted in U.S. Pat. No. 6,122,564. In this patent, an optical monitoring system is connected to a feed-back controller in order to adjust the material deposition rate depending on the indicated height of previously deposited material.
In U.S. Pat. No. 6,311,099 an apparatus for regulating laser welding parameters is suggested that uses optical signals from the interaction zone. In this patent the optical signal is generated by near infrared radiation originating from the weld pool. The radiation is detected by a CCD camera and processed in order to obtain information about the physical dimensions of the melt pool.
Previous inventions have not combined the laser metal forming process with this high degree of process control. Some superficially similar patents such as U.S. Pat. No. 4,212,900, U.S. Pat. No. 4,750,947, U.S. Pat. No. 4,015,100 do not utilise the same LMF process, and therefore suffer the disadvantages of necessitating a two step coating-melting process or of not incorporating the addition of coating material as disclosed in U.S. Pat. No. 5,659,479.
Those patents such as U.S. Pat. No. 4,644,127, U.S. Pat. No. 4,981,716, U.S. Pat. No. 5,208,431 or U.S. Pat. No. 5,889,254 utilising the powder and laser beam combination do not aim to monitor the melt condition during the process and are therefore unable to attain a comparable degree of process control. U.S. Pat. No. 5,449,536 explaining the laser deposition of a powder-based hard coating does not attempt to melt the workpiece, and instead relies on the agglomeration of heated powder particles to create a coating layer. Some previous work has used a similar coaxial laser/pyrometer signal arrangement (see U.S. Pat. No. 5,486,667). However the application in this case relates to laser cutting and machining and, moreover, the pyrometer is monitoring the plasma plume rather than the melt pool; in the present invention, which utilises solely a conduction heating regime, there is no plasma plume. In U.S. Pat. No. 5,985,056), a continuous feedback system is used to monitor “a measured parameter” and to control the laser beam power. However if used in this case, a pyrometer signal measured only the bulk workpiece temperature, rather than the melt pool, and spectroscopic measurements of process light were not used to monitor the melt pool temperature.